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TypeScript Intermediate: Types That Survive Real Projects
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TypeScript Intermediate: Types That Survive Real Projects

Check Yourself: Classes and Modules in TypeScript

Five scenario questions on access modifiers, hash fields, import type and circular imports. If you miss more than one, reread the lessons in this section.

5 min TypeScriptObject-Oriented ProgrammingAPIs and IntegrationsType Systems Intermediate
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1. A class has a private balance property. After compiling and running, console.log of an instance prints the balance, and JSON.stringify includes it. Why?
The class was compiled without strict mode. The private keyword is checked by the compiler and erased in the output, so at run time it is an ordinary property. Printing and JSON ignore modifiers only in older versions of Node. The balance was declared public by mistake.
2. The same class is written with a hash prefix on the field name instead of the private keyword. What can outside code do with it?
It can read it, but only in a browser. It can read it with bracket notation, as with private. It cannot reach the field at all, not even with bracket notation or an any cast, and console.log shows an empty object. It can read it after calling Object.keys.
3. A constructor is written as constructor with the parameter private readonly limit of type number and an empty body. What does it do?
It declares a private read-only property named limit and assigns the argument to it. It makes the class abstract. It declares limit as a local variable that disappears after the constructor. Nothing, because the body is empty.
4. Two files each import a constant from the other, and the first one reads it while loading. tsc reports no error, but running the program ends with a ReferenceError about accessing a name before initialization. What is the cleanest fix?
Turn off strict mode. Move the shared values into a third file that both import, so the dependencies point one way. Rename one of the constants. Add as any to the imports.
5. With verbatimModuleSyntax turned on, a file imports an Order that is only a type, using a plain named import. The compiler reports an error. What do you write instead?
An import of the whole file with a star. A require call instead of import. An import type line, or the type keyword in front of the name, so the compiler can see it is erased. Nothing. You turn the setting off, because the error is wrong.

About

Five scenario questions on access modifiers, hash fields, import type and circular imports. If you miss more than one, reread the lessons in this section.

Duration
5 min
Last update
10/13/2026
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